Until recently, display devices have typically used cathode-ray tubes (CRTs). More recently, considerable effort has been expanded to research and develop thin film transistor liquid crystal display (TFT-LCD) devices having thin profiles, light weight and low power consumption as substitutes for CRTs.
Liquid crystal display (LCD) devices use the optical anisotropy and polarization properties of liquid crystal molecules of a liquid crystal layer to produce an image. The liquid crystal molecules have long and thin shapes. Because of the optical anisotropy property, the polarization of light varies with the alignment direction of the liquid crystal molecules. The alignment direction of the liquid crystal molecules can be controlled by varying the intensity of an electric field applied to the liquid crystal layer. Accordingly, a typical LCD device includes two substrates spaced apart and facing each other and a liquid crystal layer interposed between the two substrates. Each of the two substrates includes an electrode on a surface facing the other of the two substrates. A voltage is applied to each electrode to induce an electric field between the electrodes. The arrangement of the liquid crystal molecules as well as the transmittance of light through the liquid crystal layer is controlled by varying the intensity of the electric field. LCD devices are non-emissive type display devices that employ a light source to display images using the change in light transmittance.
Among the various types of LCD devices, active matrix LCD (AM-LCD) devices that employ switching elements and pixel electrodes arranged in a matrix structure are the subject of significant research and development because of their high resolution and superior suitability for displaying moving images. Thin film transistor LCD (TFT-LCD) devices use thin film transistors (TFTs) as the switching elements.
FIG. 1 is a perspective view of an LCD device according to the related art. As shown in FIG. 1, the LCD device of the related art includes a first substrate 10, a second substrate 20 and a liquid crystal layer 30. The first substrate 10 is referred to as an array substrate and includes a gate line 14 and a data line 16 crossing each other to define a pixel region P. A pixel electrode 18 and a thin film transistor (TFT) Tr, as a switching element, are positioned in each pixel region P. Thin film transistors Tr, which are disposed adjacent to crossings of the gate lines 14 and the data lines 16 are disposed in a matrix on the first substrate 10. The second substrate 20 is referred to as a color filter substrate, and includes color filter layer 26 including red (R), green (G) and blue (B) color filters 26a, 26b and 26c, a black matrix 25 between the red, green and blue color filters 26a, 26b and 26c and a common electrode 28 on both the color filter layer 26 and the black matrix 25.
Although not shown in FIG. 1, the first and second substrates 10 and 20 are attached with a seal pattern to prevent leakage of liquid crystal layer 30. In addition, a first alignment layer is formed between the first substrate 10 and the liquid crystal layer 30 and a second alignment layer is formed between the second substrate 20 and the liquid crystal layer 30 to align the liquid crystal molecules in the liquid crystal layer 30 along an initial alignment direction. A polarization plate is formed on an outer surface of at least one of the first and second substrates 10 and 20.
Further, a backlight unit (not shown) disposed under the first substrate 10 supplies light. A gate signal turning the TFT Tr on is sequentially applied to each of the gate lines 14, and an image signal on the data line 16 is applied to the pixel electrode 18 in the pixel region P. The liquid crystal molecules in the liquid crystal layer 30 are driven by a vertical electric field generated between the pixel electrode 18 and the common electrode 28 to display images by varying the light transmittance of the liquid crystal molecules.
In the above related art LCD device, elements of a metallic material such as the gate line 14, the gate electrode (not shown), the data line 16, the source electrode (not shown), the drain electrode (not shown) and the pixel electrode 18 may be formed through a physical vapor deposition (PVD) method such as a sputtering. Elements of an inorganic insulating material or a semiconductor material such as the gate insulating layer (not shown), the passivation layer (not shown) and a semiconductor layer may be formed through a chemical vapor deposition (CVD) method. In addition, the CVD method requires a temperature higher than about 300° C. Since a glass substrate having a transition temperature of about 600° C. is used as the first and second substrates 10 and 20 in the related art LCD device, the elements through the CVD method or the PVD method may be formed on the first and second substrates 10 and 20 without any problems.
Recently, however, as a portable terminal having a small size such as a notebook and a personal digital assistant (PDA) is widely used, an LCD device having light weight and flexibility is required for applying to the portable terminal. As a result, an LCD device including first and second substrates formed of transparent plastic has been researched and developed. However, since plastic is inferior to glass in thermal resistance and chemical resistance, a plastic substrate has disadvantages in a fabrication process for an LCD device including a high temperature process over about 200° C., specifically, in a fabrication process for an array substrate having a TFT. For example, when an inorganic insulating layer and a semiconductor layer are formed on a substrate through a CVD method, a maximum temperature may be at least 300° C. Accordingly, the plastic substrate may be deformed or degraded, and there exist many problems in production of an LCD device having a plastic substrate. Further, a produced LCD device having a plastic substrate may have a poor display quality due to the deformation or degradation of the substrate.
Although an organic semiconductor material which can be coated on a substrate at room temperature has been researched for the semiconductor layer, the inorganic insulating layer is still formed through a CVD method. The inorganic insulating layer formed through a CVD method causes deformation or degradation of a plastic substrate.